Method for developing and fixing electrostatic images in initially partially cured base elements



June'zs. 1965 K. A. METCALFE Em 3 192,043

METHOD FOR DEVELOPING AND FIXING ELECTROSTATIC IMAGES IN INITIALLY PARTIALLY CURED BASE ELEMENTS Filed Oct. 3, 1961 United States Patent METHOD FOR DEVELOPING AND FIXING ELEC TROSTATIC IMAGES IN INITIALLY PARTIALLY CURED BASE ELEMENTS Kenneth Archibald Metcalfe, Fulham, South Australia, and Robert John Wright, Hectorville, South Australia, Australia, assignors to The Commonwealth of Australia, Melbourne, Victoria, Australia Filed Oct. 3, 1961, Ser. No. 142,621 Claims priority, application Australia, Oct. 7, 1960,

9 Claims. (Cl. 961) This invention relates to an improved base element and developer for electrostatic printing and more particularly to methods and means of electrostatic image formation and fixing of selected areas of a surface.

An electrostatic printing process is that type of process for producing a' visible record, reproduction or copy by i converting a light image or electric signal into an electrostatic charge pattern on an electrically-insulating base. The process includes the conversion of the electrostatic charge pattern into a visible image by development with suitable charged particles.

A typical electrostatic printing process uses an insulating base coated with a suitable photoconductor which on charging produces an overall blanket electrostatic charge on the surface. Such electrostatic charge may be stored in the dark but slowly dissipates with time. During the period inwhich a substantial charge remains, a latent electrostaticimage can be formed by exposing the charged surface to electromagnetic waves which discharge the non-image areas according to the original image leaving the remainder of the surface in a charged condition. The electrostatic image is made visible by applying charged particles in a suitable liquid developer. The visible image as formed is at present fixed to the surface by a fixing agent incorporated in the developing liquid.

When using powder developers as well known before liquid development processes were invented, it was customary to have a photoconductor surface onto which the image was deposited selectively during development, and to incorporate in the developer a medium which was subsequently fused to fix the image to the base, the photoconductor material for this purpose being embedded in an insulating film which was itself a thermoplastic substance so that by the application of the heat the image was fused, the surface of the base being softened to allow the image to be effectively bonded thereto.

With the advent however of liquid development, the need of providing fusing of the image to fix it was removed because the liquid developer then carried the fixiice According to this invention the fixing of the electrostatic latent-image is accomplished by using a coating of a suitable substance on the base material which is not fully cured and softens by immersing the said base in the developer liquid. The developer particles are thus secured to the surface by incorporating the said particles into the softened coating.

One type of base element usable in the foregoing electrostatic printing process comprises a cellulose type sheet coated with a finely divided or particulate photoconductor. dispersed in an electrically-insulated solid film forming vehicle, for example ordinary paper coated with particulate photoconducting white-zinc oxide dispersed in a-resin. It was thought that such recording elements produce satisfactory prints only if the resin binder was cured by time or accelerated by temperature" and that poor prints result if the element wasnot fully cured, but it has now been-found that partial curing (preferably in excess of about percentof ,complete curing) will operate and this then allows softening of the resin by immersion in" the developer liquid and so affords coupling of the developer particle with the resin coating.

A necessary feature of this invention is therefore an incomplete curing of the resin coating is made and this allows a softening of the resin face on immersion in the developer liquid. This condition leads to a coupling of the dispersed developer particle suspended in the said liquid to form a combination which does not make it necessary to have a fixing agent in the developer liquid with the suspended developer particles. 7

Curing should be sufficiently complete to ensure that surface softening only of the bonding material takes place without removal of appreciable amounts of the bonding material and consequent release of therphotoconductor medium. i f

In order that the invention may be fully understood an embodiment thereof will now be described with reference to'the accompanying drawings, in which:

FIG. 1 is a schematic cross section of a base element.

FIG. 2 is a similar cross section but showing how liquid development is applied to such base element, and

ing medium, and the developer itself was then capable of fixing the image in that the fixing medium bonded the pigment particles to the base by adhesive action.

Liquid developer therefore provided the direct means of fixing the image immediately after the carrier liquid was evaporated, the carrier for this purpose having a solvent effect on the fixing substance to ensure that while the developer was in its liquid state, the fixing agent was maintained in a condition where it could effect the necessary bonding.

The present invention envisages certain improvements to the fixing of the image, the improvements relating to the surface of the base element itself which achieves an effect somewhat analogous to the heat fusion methods FIG. 3 is a view showing the base element with the image thereon. V

The base element comprises a sheet of paper 1 or similar medium on the surface of which is formed a conductive layer 2 and on which in turn is a film 3 comprising a bonding medium 4 and a photoconductor 5, the photoconductor being dispersed in the bonding medium 4.

The liquid developer 6 comprises a carrier liquid 7 hav- 7 ing an electrical volume resistivity of not less than about 10 ohm centimetre and a dielectric constant of at least 3 so that the liquid will not destroy the electrostatic image on the photoconductor during development, and in this carrier liquid are dispersed the pigment particles 8 which are s'oselected that they will deposit preferentially according to the charge on the photoconductor surface.

The carrier liquid 7 is such that it is capable of dissolving the, film 3 at the interface 9, provided the bonding medium 4 has not been fully cured, and according to this invention, therefore the film 3 is cured only sufiiciently to stabilise it to ensure that the image from the photoconductor medium will not be bled away prior to development, this being generally achieved by carrying the curing to at least half way, or somewhat less than what would normally be the fully cured condition of the bonding medium. i i i According to this invention therefore the base element comprises a backing of paper or the like on which is a'conductive layer 2 such as a layer of aluminum which is vacuum coated or sputtered on to the backing 1 and on this is the film 3 consisting of a bonding medium 4-and a I) photoconductor in which the bonding medium is at least one half cured, the developer used therewith consisting of pigment particles 8 suspended in a carrier liquid 7, which liquid 7 is a solvent for the partially cured bonding medium 4 of the base element.

Examples of the way in which for purposes of this invention, a base material is formulated and treated and combined with a developer designed to effect softening of the surface of the material during the course of development are as follows:

Example 1 In one example of our invention coating material was prepared according to the following formulation:

Grams Alkyd resin, Rhodene L59/ 60 100 Zinc oxide, Durham Microx, a product of Durham Chemicals Ltd. 180 Driers:

4% cobalt naphthenate 0.5 lead naphthenate 1.0 Toluene 200 Rhodene L59/ 60 is the trademark of the Polymer Corporation Pty. Ltd. for a rosin phenolic resin modified linseed-wood oil alkyd resin.

These constituents were milled together to form a paint by milling a ball mill for 8 hours or a single roll mill for 1 hour. paper or sheet of metal by conventional coating means such as dip or knife coating, by spraying or by electrostatically coating. After application the coating was cured for 1 hour at 140 F.

The next step was to give the coating an electrostatic charge over the surface by subjecting it to the corona discharge from points or wires maintained at high potential relative to the web on which the coating had been made, this operation being carried out in relative darkness.

The overall surface charge was dissipated imagewise by exposing the coating to light or X-ray image from a suitable source either by contact or shadow printing or by projection printing or by exposure in a conventional camera.

Following production of an electrostatic latent image development of the image was effected by subjecting the surface of the coating to a liquid-based developer having the following composition:

Pigment, for example, Microlith Black CT, an organic pigment produced by Ciba Coy. Switzerland gram 1 Carrier liquid, Shellite cc 500 Shellite is the trademark for a hydrocarbon solvent. These constituents were ground together in a pestle and mortar to disperse the pigment in the carrier liquid. The resultant dispersion was used as a developer, the pigment being attracted to the charged areas of the image. The coating material cited in the first part of this example softened slightly in the carrier liquid and upon evaporation of this liquid the pigment was secured to the surface of the coating.

Example 2 In another example of our invention, the coating material of the first part was prepared according to the following formulation:

Silicone resin, Dow Corning No. 804 grams 480 Zinc oxide, Durham Special --do 1200 Toluene millilitres 500 These constituents were ground together in a ball mill or in a single roll mill to secure dispersion of the resin and oxide in the form of a paint. For example, the constituents were milled for 8 hours in a ball mill or for 1 hour in a single roll mill. The resultant paint was diluted with a further 500 millilitres of toluene to produce a satisfactory coating consistency, for example for knife coat- 4. ing, spray coating or dip coating. Curing was effected b heating to 200 F. per hour.

The coating was then charged and exposed as described in Example 1, following which the latent electrostatic image was developed with a developer of the following composition:

Pigment, for example, carbon black, Peerless black gram 1 Carrier liquid, cyclohexane millilitres 500 These constituents were ground together to form a dispersion which was then used as a developer. The coat- 7 ing cited in the first part of this example softened slightly The resultant paint was applied to a web of in the cyclohexane carrier liquid and upon evaporation of this liquid the pigment was secured to the surface of the coating.

Example 3 In a third example of our invention the coating material used had the following composition:

Polystyrene grams 500 Zinc oxide, Durham Special do 1200 Toluene mi1lilitres 500 Styrene monomer do 500 These constituents were ground together in a ball mill for 8 hours or in a single or triple roll mill for 1 hour following which the resultant paint was applied by conventional coating means to a web of paper or to a sheet of metal. After drying for 1 hour at 160 F. the coating was subjected to the charging and exposure steps previously described.

Development of the latent image on the coating of this example was achieved by means of a liquid developer having the following composition:

Pigment, Lithol Red "gram" 1 Solvent X4, Shell Coy millilitres 500 Cyclohexane do 500 Freon 11 do 500 Freon 11 is the trademark of Du Pont de Nemours Corp. for a trichloromonofluoromethane (CCl F).

The coating given in the first part of this example softened sumciently in the carrier liquid so that after evaporation the pigment was fixed to the coating.

Example 4 In a further example of this invention a coating material was made which ordinarily if fully cured would not soften in any of the liquids commonly used for liquid development. This formulation was as follows:

Grams Alkyd resin, L9/50 Rhodene Zinc oxide, Durham Microx 180 Toluene 200 Rhodene L9/50 resin is the registered trademark of the Polymer Corporation Ltd. Australia, for a linseed oil modified alkyd resin.

When made up as a paint and applied as a coating this paint in the absence of driers even though cured for 16 hours at F. was softened by Shellite, Solvesso 150, and the like when used as liquid developers. By inhibiting the curing of the resin therefore the invention secures softening of the coating by the carrier liquid in order to obtain fixing.

With reference to Example 2, the control of softening obtainable in this invention is shown by the fact that at 200 F. fixing was obtained but at 400 F. for one hour none of the common carrier liquids would soften the coating. i i

What we claim is:

1. An improved method of developing and simultaneously fixing electrostatic images involving the use of a volatile liquid comprising producing an electrostatic image on a photoconductor embedded in 1a fusible polymeric bonding material on a base, said bonding material being partially cured to the degree that it is soluble to a limited extent in said liquid, then developing said electrostatic image by applying to the surface of the bonding material a developer which comprises a pigment suspended in said volatile liquid, said liquid being of an electrical resistivity sufiiciently high to prevent destruction of the electrostatic image during development, the bonding material being softened due to its solubility in said liquid to enable the pigment to penetrate the bonding material upon deposition thereon, at least some of the pigment being totally embedded in said bonding material, and then evaporating the carrier liquid of the developer to harden the bonding medium to hold the pigment.

2. An improved method of developing and simultaneously fixing electrostatic images involving the use of a volatile liquid and a fusible polymeric bonding medium which in less than fully cured condition is soluble in said liquid, said method comprising embedding a photoconductor medium in said bonding medium which is positioned on a base, partially curing the said bonding medium to at least half of its fully cured condition whereby it remains soluble in said liquid, producing an electrostatic image on the photoconductor medium embedded in said partly cured bonding material by charging and exposing to a light image, then applying said volatile liquid to the said bonding medium, said liquid having an electrical resistivity sufiiciently high to prevent destruction of the electrostatic image during development and hav ing dispersed therein a pigment attractable to said image, the bonding medium being softened by the liquid to enable deposition pigment to penetrate the bonding medium under deposition thereon, at least some of the pigment being totally embedded in said bonding material, and then evaporating the liquid to harden the bonding medium to hold the pigment.

3. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding material of the base element is a resin and the carrier liquid of the developer is a hydrocarbon solvent.

4. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding material of the base element is a silicone resin and the carrier liquid of this developer is cyclohexane. V

5. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding medium of the base element comprises polystyrene.

6. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding medium of the base element comprises polystyrene and the carrier liquid is a hydrocarbon.

7. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding medium of the base element comprises polystyrene and the carrier liquid is cyclohexane.

8. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding medium of the base element comprises polystyrene and the carrier liquid is trichloromonofluoromethane.

9. An improved method using a base element and developer for electrostatic printing processes according to claim 1 wherein the bonding medium of the base element is an alkyd resin and the carrier liquid of the developer is a hydrocarbon liquid.

References Cited by the Examiner UNITED STATES PATENTS 2,579,332 12/51 Nelson.

2,735,784 2/56 Greig et al. 96-1 2,857,271 10/58 Sugarman 1l717.5 X 2,862,815 12/58 Sugarman et al. 1l7--17.5 X 2,907,674 10/59 Metcalfe et al. 11737 2,946,682 7/ Lauriello 961 2,954,291 9/60 Clark 96-1 2,959,481 11/60 Kucera 961 2,993,787 7/61 Sugarman.

3,010,842 11/61 Ricker 11737 3,041,168 6/62 Wielicki 117--l7.5 X 3,043,684 7/62 Mayer 96--1 NORMAN G. TORCHIN, Primary Examiner. WILLIAM D. MARTIN, Examiner, 

1. AN IMPROVED METHOD OF DEVELOPING AND SIMULTANEOUSLY FIXING ELECTROSTATIC IMAGES INVOLVING THE USE OF A VOLATILE LIQUID COMPRISING PRODUCING AN ELECTROSTATIC IMAGE ON A PHOTOCONDUCTOR EMBEDDED IN A FUSIBLE POLY MERIC BONDING MATERIAL ON A BASE, SAID BONDING MATERIAL BEING PARTIALLY CURED TO THE DEGREE THAT IT IS SOLUBLE TO A LIMITED EXTENT IN SAID LIQUID, THEN DEVELOPING SAID ELECTROSTATIC IMAGE BY APPLYING TO THE SURFACE OF THE BONDING MATERIAL A DEVELOPER WHICH COMPRISES A PIGMENT SUSPENDED IN SAID VOLATILE LIQUID, SAID LIQUID BEING OF AN ELECTRICAL RESISTIVITY SUFFICIENTLY HIGH TO PREVENT DESTRUCTION OF THE ELECTROSTATIC IMAGE DURING DEVELOPMENT, THE BONDING MATERIAL BEING SOFTENED DUE TO ITS SOLUBILITY IN SAID LIQUID TO ENABLE THE PIGMENT TO PENETRATE THE BONDING MATERIAL UPON DEPOSITION THEREON, AT LEAST SOME OF THE PIGMENT BEING TOTALLY EMBEDDED IN SAID BONDING MATERIAL, AND THEN EVAPORATING THE CARRIER LIQUID OF THE DEVELOPER TO HARDEN THE BONDING MEDIUM TO HOLD THE PIGMENT. 